Method for characterizing a structure on a mask and device for carrying out said method

ABSTRACT

A method is provided for characterizing a mask having a structure, comprising the steps of: —illuminating said mask under at least one illumination angle with monochromatic illuminating radiation, so as to produce a diffraction pattern of said structure that includes at least two maxima of adjacent diffraction orders, —capturing said diffraction pattern, —determining the intensities of the maxima of the adjacent diffraction orders, —determining an intensity quotient of the intensities. A mask inspection microscope for characterizing a mask in conjunction with the performance of the inventive method is also provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the National Stage of International ApplicationPCT/EP2011/006278, filed on Dec. 13, 2011, which claims priority toGerman Application DE 10 2010 056 407.9, filed on Dec. 23, 2010, andU.S. Provisional Patent Application 61/426,648, filed on Dec. 23, 2010.The contents of the above applications are incorporated by reference intheir entireties.

BACKGROUND

This patent specification is directed to a method for characterizing astructure on a mask.

This patent specification is also directed to a device for carrying outsaid method.

In the fabrication of semiconductor components, numerous measurementmethods are used to monitor the results of the individual steps in thelithographic process. One fundamental task in this context is to measurethe outer dimensions or shapes of the structures which are to beproduced.

To check the quality of a process step, it is advantageous to examinethe smallest imageable structures. These are referred to as the criticaldimension, abbreviated CD. The critical dimension of a mask usuallyrefers to the line width of a structure composed of alternating linesand spaces. In the lithographic process, this structure is first formedon a mask (photomask, reticle), for example in chromium on quartz glass.There are also other known masks, for example phase shift masks (PSM) orreflective masks, that are used particularly with short-wavelengthilluminating radiation in the EUV range. By exposure in a scanner, thisstructure is imaged onto a wafer coated with resist. The desiredstructure is produced on the wafer by subsequent developing and etching.

The characterization of a structure, such as, for example, measurementof the CD or line width of a structure, can be performed both on themask and on the wafer. On-wafer measurement does yield very meaningfuldata, since the final product is measured; it is very laborious,however, since the entire wafer exposure process has to be performed forthe test.

If the characterization of a mask structure is performed on the maskitself, errors caused by the behavior of the mask during imaging and byother method steps are incorporated into the measurement. Errors on themask are usually intensified by the scanner during imaging. Anotherproblem is that the mask structures are known and are optimized byresolution enhancement technology (RET), and thus do not wholly matchthe structures that are to be imaged. This makes it difficult to measurethe CD on the mask.

Both on mask and on wafer, CD measurement is performed by, for example,scanning electron microscopy (CD-SEM, critical dimension scanningelectron microscopy).

Another option for characterizing masks is to analyze the aerial imagesof masks with a mask inspection microscope. In this method, the aerialimage shows most of the features that are also projected onto the wafer.

Another option for characterizing masks and wafers is afforded bynon-imaging optical methods. Various interrelated measurement methodsare used for this purpose, such as, for example, conventionalscatterometry, ellipsometry, diffractometry or reflectometry, knowncollectively under the generic term scatterometry. Masks can also becharacterized by measuring mask transmission. The CD of a structure canalso be determined by this method, as disclosed, for example, in EP2171539 A1.

SUMMARY OF INVENTION

The object of the invention is to provide a fast and inexpensive methodwith high measurement accuracy and a device for characterizing astructure on a mask.

According to the invention, this object is achieved by means of a methodfor characterizing a mask having a structure, comprising the steps of:illuminating the mask with monochromatic illuminating radiation under atleast one angle of illumination, so as to obtain a diffraction patternof the structure that includes at least two maxima of adjacentdiffraction orders; capturing the diffraction pattern; determining theintensities of the maxima of adjacent diffraction orders; determining anintensity quotient of the intensities.

This measure permits the rapid determination of an accurate measurementvalue for characterizing the structure of a mask. Since the quotient ofthe maxima of adjacent diffraction orders is calculated, variations inthe intensity of the exposure strength have little or no effect on thismethod of measurement.

Determining the intensities of the diffraction order maxima from thecaptured diffraction patterns is a much simpler process than maskstructure characterization based on aerial images of the mask structure.This reduces the computation time spent on image analysis by the dataprocessing system used.

An additional advantage over the analysis of aerial images is that thecharacterization is averaged over the detail under examination.

If different diffraction maxima of different structures appear in thearea under examination, however, the analysis can easily be limited tothe two relevant diffraction maxima. The relevant diffraction orders canbe determined in a simple manner by image processing.

In a further embodiment of the invention, the diffraction pattern isprojected entirely onto a single detector for capture.

This measure has the advantage that a complete diffraction pattern isobtained quickly. Lateral movement of the detector is not necessary.

One advantage over the capture and analysis of aerial images is that thecapture of diffraction patterns is much more tolerant of defocusing ofthe diffraction pattern. This makes it possible to perform a great manymeasurements at different positions on a mask without having to focusbetween individual measurements. This considerably increases the speedof characterization of a mask.

In a further embodiment of the invention, the diffraction pattern isbeing captured during a continuous relative movement between the maskand the detector.

In a version of this embodiment the continuous relative movement isaccomplished by movement of the mask. In a further version of thisembodiment the mask is placed on a mask holder to enable a continuousrelative movement of the mask. The mask will e. g. moved in a planeparallel to the plane of the detector.

In a version of this embodiment the speed of movements and the exposuretime of the detector is specified in a way that each portion of theregion to be characterized contributes to the final diffraction patternto the same extend.

This measure has the advantage that a fast characterization of a wholemask or of a large connected part of a mask is possible. Scanning ofmasks by capturing aerial images of the mask structure is more complex.Therefore single aerial images of the mask structure in sections have tobe captured. But this requests short exposure times or slow scanningspeeds. Alternatively more sophisticated detectors could be used, thatallow a time delayed integration of the signals. So called TDI (timedelayed integration)-detectors are used. Performing these methodscharges of the detector are moved synchronously with the scanningmovement to accomplish a long exposure time.

In a further embodiment of the invention, a line width of the structureis determined from at least one correlation between at least onedetermined intensity quotient of a structure and a known line width ofthat structure.

This measure has the advantage that absolute values can be accessed in asimple manner. The calibration can be performed against measurements ofthe structure on the mask or on the exposed wafer or against an aerialimage of the mask.

In a further embodiment of the invention, the illuminating radiationproduces on the mask an illumination field having a main region and anedge region, said edge region surrounding said main region, wherein theintensity of the illuminating radiation is constant in the main regionbut decreases continuously in the edge region.

This measure has the advantage of reducing the dependence of thedetermined intensity quotient on changes in the position of theto-be-characterized mask structure in the illumination field.

Mask inspection microscopes known from the prior art employ anillumination field that has a constant intensity distribution over theentire region, with the intensity decreasing almost abruptly to zero atthe edge of this illumination field. Such intensity distributions areknown as “top hat profiles.”

A disadvantage associated with the use of these intensity distributionsis that the determined intensity quotients of a structure can depend onthe position of that structure in the illumination field. For example,in the case of a structure configured as “lines and spaces,” of the kindmentioned at the beginning hereof, the parts of the structure in theillumination field that contribute to the diffraction of theilluminating radiation vary. This variation would depend on the positionof the structure in the illumination field along the grating period ofthe structure. This variation causes a change in the determinedintensity quotients. This leads to problems in the comparison ofidentical types of structures that are expected to yield identicalintensity quotients. The reproducibility of the positioning ofstructures in the illumination field is limited. For the above reasons,positioning errors can cause errors in the intensity quotients that areto be determined.

In a further embodiment of the invention, the decrease in intensity inthe edge region corresponds to a Gaussian function.

This measure has the advantage that the dependence of the intensityquotients on variations in the position of the mask structure in theillumination field is especially low.

In a further embodiment of the invention, the illumination field on themask is defined by means of a field stop and the intensity distributionof the illumination field is adjusted by defocusing the field stop.

This measure has the advantage that a desired intensity distribution ofthe illumination field can be specified in a simple manner. Theintensity distribution obtained by defocusing the field stop correspondsto that of the above-cited measure, in which the intensity of theilluminating radiation is constant in the main region of theillumination field, but decreases as a Gaussian function in the edgeregion.

In a further embodiment of the invention, the at least one illuminationangle is adjusted to a grating period of the structure.

In this case, the illumination angle is adjusted in the direction of thenormal to the surface of the mask, the z-axis, and/or in the directionof the surface of the mask, i.e., the azimuth angle.

This measure has the advantage that the diffraction maxima of theto-be-examined structures, to which the at least one illumination anglehas been adjusted, are particularly sharply defined in the diffractionpattern. Adjusting the illumination angle has the effect that thediffraction orders to be analyzed are captured with the highest possibleintensity or contrast, largely without any interference effects fromother structures. This permits simple and precise analysis of thediffraction pattern.

In a further embodiment of the invention, the at least one illuminationangle is adjusted to an orientation of the grating period of thestructure on the mask.

In this measure, the illumination angle is adjusted in the direction ofthe surface, to the azimuth angle of the illuminating radiation. Thesurface of the mask is described by a right-angle coordinate systemhaving an x- and a y-axis. For example, structures whose grating periodsextend in the x- or y-direction can be disposed on masks. Thesestructures are configured as lines and spaces, for example. If thegrating period of a structure on a mask extends in the direction of thex-axis, then the structure is designated an x-structure. If the gratingperiod extends in the direction of the y-axis, then the structure isdesignated a y-structure. The azimuth angle of the illuminatingradiation is 0° in the case of x-structures and 90° in the case ofy-structures.

Comparing the intensity quotients of different positions on a mask isespecially meaningful for characterization when measurements ofcomparable structures, i.e., structures having the same nominal gratingperiod and line width, are compared with one another. For instance, x-and y-structures having the same grating period and line width are anexample of comparable structures.

This measure has the advantage that adjusting the illumination angle tothe structures makes it possible to image the diffraction maxima ofcomparable structures with the highest possible contrast.

Since possible positions of the diffraction maxima in a captureddiffraction pattern are specified by means of the at least one adjustedillumination angle, the analysis is further simplified.

In a further embodiment of the invention, the structure on the mask isilluminated simultaneously under at least two illumination angles, eachof the illumination angles being adjusted to the orientation of therespective grating periods of different structures on the mask.

This measure has the advantage that structures having at least twoorientations can be captured simultaneously. In this case, for example,the above-mentioned x- and y-structures present on masks can be examinedtogether. This eliminates the need to change the illumination directionbetween measurements of x- or y-structures.

In a further embodiment of the invention, the at least one illuminationangle is predefined by arranging a stop in a pupil plane of anillumination beam path.

This measure has the advantage that the illumination angle can bespecified in a simple manner. The stops of a mask inspection microscopecan be changed quickly and easily. The shape of the stop can bemanipulated to yield almost any arbitrary illumination angledistribution.

This measure also has the advantage that an imaging method employing amask inspection microscope can be used in rapid alternation with thecapture of diffraction patterns. Thus, for example, both the diffractionpattern and the aerial image of a structure on a mask can be capturedand analyzed.

In a further embodiment of the invention, the stop is configured as anannulus that is adjusted to the grating periods of different structureson the mask.

This measure has the advantage that optimal illumination angles can bespecified for structures within a grating period or correspondingregion, regardless of the orientation of the grating period on the mask.

In a further embodiment of the invention, the stop is configured as amonopole, the position of the pole being adapted to the orientation ofthe grating period of a structure on the mask.

This measure has the advantage that an optimum illumination angle can bespecified for structures within a grating period or corresponding regionand having a corresponding orientation of the grating period on themask. In this case, for example, the above-cited x- or y-structures canbe examined individually.

In a further embodiment of the invention, the stop is configured as anasymmetric dipole, the positions of the two poles being adapted to theorientations of the grating periods of different structures on the mask.

This measure has the advantage that the simultaneous illumination of thestructure of the mask at two angles of illumination, as noted inconnection with a previously cited exemplary embodiment, is easy torealize.

In a further embodiment of the invention, the intensities of theadjacent diffraction orders are normalized to a reference value,particularly to the intensity, measured during the characterization of amask with no structure.

This measure has the advantage that the normalized intensities of themaxima of identical diffraction orders are comparable. This makespossible a further characterization of the structure on the mask. Byanalyzing the normalized intensities of the diffraction maxima,comparable structures can be identified in the diffraction pattern. Inaddition, comparing the normalized intensities of the diffraction maximaof different diffraction patterns furnishes a criterion for determiningdiffraction patterns of comparable structures.

In a further embodiment of the invention, the assignment of the maximaof two adjacent diffraction orders of a diffraction pattern to astructure is performed by applying at least one of the criteria:position of the diffraction order maxima, distance between thediffraction order maxima, intensities of the diffraction order maxima,spread of the diffraction order maxima.

This measure has the advantage that a further characterization of thestructures can be done quickly and easily on the basis of thediffraction patterns. For example, it becomes possible to assign themaxima of two adjacent diffraction orders in a diffraction pattern to astructure. For example, lines and spaces of a specific grating periodand line width can be detected.

In another embodiment of the invention, a further characterization ofthe structure is performed by comparing the captured diffraction patternto a simulated diffraction pattern.

The diffraction pattern of the structure to be characterized can besimulated to the extent that the design of the mask is known. Deviationsof the captured diffraction pattern from the simulated one are evidenceof deviations of the structure on the mask from the structure specifiedin the design.

This measure has the advantage that the captured diffraction pattern canalso be used to perform a comparison with the specified structure of themask.

In a further embodiment of the invention, plural positions on the maskat which structures are characterized are specified.

This measure has the advantage that masks can be characterized rapidly.

In a further embodiment of the invention, plural positions evenlydistributed over the surface of the mask are specified.

This measure has the advantage that masks can be characterized even whenthe available information on the structures is nonexistent orincomplete. This embodiment is particularly advantageous in combinationwith the cited embodiments for locating comparable structures.

In a further embodiment of the invention, comparable structures areidentified in the diffraction patterns by applying at least one of thecriteria: position of the diffraction order maxima, distance between thediffraction order maxima, intensities of the diffraction order maxima,spread of the diffraction order maxima, differences from simulateddiffraction patterns.

This measure has the advantage that comparable structures can be locatedin a simple manner.

In a further embodiment of the invention, positions at which comparablestructures are formed on the mask are specified.

This measure has the advantage that comparable structures can beselectively chosen. This eliminates unnecessary measurements and theneed to select diffraction patterns of comparable structures from alarge number of captured images. The specification of positions can becarried out particularly on the basis of the mask design.

In a further embodiment of the invention, the mean of the intensityquotients of all comparable structures and the percentage deviation ofthe individual intensity quotients from the mean are calculated.

This measure has the advantage that simple and meaningful assessment ofthe mask becomes possible. Given the usually small deviations of theline widths from the nominal value, the values obtained correspond ingood approximation to the deviations of the critical dimension (CD) fromthe nominal value.

The invention also encompasses a mask inspection microscope comprising adata processing system that performs the steps of the method accordingto the invention.

The data processing system is, for example, a commercially availablecomputer programmed in such a way that all the aforesaid methods andtheir embodiments can be carried out.

It is understood that the features of the invention cited above andexplained in more detail below can be used not only in the describedcombinations, but also in other ones, without departing from the scopeof the present invention.

BRIEF DESCRIPTION OF DRAWINGS

The invention will be described and explained in more detail below onthe basis of a few selected exemplary embodiments and by reference tothe drawings.

Therein, the figures illustrate:

FIG. 1: a longitudinal section of a segment of an illumination beam pathof a mask inspection microscope according to the invention;

FIG. 2: a longitudinal section of a segment of an illumination beam pathand imaging beam path of a mask inspection microscope according to theinvention;

FIG. 3: a schematic representation of a mask inspection microscopeaccording to the invention employing illuminating radiation in the EUVrange;

FIG. 4: a plan view of a stop plate with various stops;

FIG. 5: the intensity distribution of an illumination field;

FIG. 6: a flow chart of an example of the method according to theinvention.

DETAILED DESCRIPTION OF THE INVENTION

A mask inspection microscope according to a first exemplary embodimentconsists, as illustrated in FIG. 1, of a radiation source 5, an excimerlaser, emitting illuminating radiation with a wavelength of 193 nm. Thisis followed along the optical axis 1 by a homogenizer 10, forhomogenizing the intensity distribution of the illuminating radiation inthe pupil plane and for the depolarization thereof.

Next comes the stop plate 45, which is disposed in a pupil plane of theillumination beam path 3. Control is effected via drive 50, which makesit possible to position the stops 51 precisely.

The selected stop of stops 51 is projected in the desired size onto thepupil plane 135 of the condenser 130 by means of a zoom lens 55 equippedwith an adjusting drive 60, as illustrated in FIG. 2. The imaging scalecan be varied by a factor of 3. A stop plate is depicted in FIG. 4; theregions opaque to illuminating radiation are illustrated as shaded. Ifthe stops are configured as reflective stops, the shaded regions are notreflective to illuminating radiation. The stop plate 45 is removablyconnected to the drive 50.

The continuation of the illumination beam path 3 from the firstexemplary embodiment is illustrated in FIG. 2. The next element is afield stop 100 for defining the size and intensity distribution of theillumination field 240, which is sketched in FIG. 5, on the mask 145.Stop 100 is imaged onto the mask. A drive 101 serves to focus anddefocus the image of the stop 100 on the mask.

The square opening of the field stop 100 has an edge length of 340 μm.After focusing on the mask, this yields an illumination field with anedge length of 26 μm. The use of different-sized field stops is providedfor. In another variant not illustrated in the figures, no field stop100 is used.

An example of a selectable illumination field 240 is sketched in FIG. 5.The intensity distribution of the illuminating radiation within thesquare illumination field 240 is constant within a square main region241. The main region 241 is smaller than the illumination field 240 as awhole, and is surrounded by an edge region 242 of constant width. In theedge region, the intensity of the illuminating radiation decreasescontinuously to the edge of the illuminating field. The decrease inintensity can be described in the form of a Gaussian curve. Theintensity distribution of the illuminating field 240 is plotted as acurve 243 along an axis denoted x. The x-axis bisects the illuminationfield 240 parallel to two opposite outer edges.

The field stop 100 is followed by a tube lens 105 and the condenser 130,which has a pupil plane 135.

To polarize the illuminating radiation, the polarizers 110 and 120 canbe brought into the illumination beam path 3 by means of the drives 115and 125. Polarizer 110 polarizes the illuminating radiation linearly;the direction of polarization can be adjusted by rotating the polarizer110 by means of drive 115. To achieve tangential polarization of theilluminating radiation, polarizer 120, which is configured as asegmented polarization converter, is brought into the illumination beampath 3 in addition to polarizer 110. The linear polarization is rotatedin sectors by this polarizer 120, thus resulting in approximatelytangential polarization. Three variants of polarizer 120 are available(not shown in FIG. 2). Division into 4, 8 or 12 sectors can be chosen.

The mask 145 to be inspected, with the structure 150, is protected by apellicle 155. The mask rests on the mask holder 140, which is moved bymeans of drive 142 laterally in a plane, denoted the xy plane, in orderto move the mask to the desired position such that the location to beinspected is in the illumination beam path 3. Via drive 142, the mask isalso moved for focusing purposes in the direction of the optical axis,the z-axis. The image of the mask is projected by objective 160 via tubelens 165, field stop 170, magnification optic 175 to the detector 200, aCCD (charged coupled device) chip. The numerical aperture is adjusted bymeans of the NA stop 180 with drive 182.

To image the pupil plane of the illumination beam path 3 onto thedetector 200, a Bertrand lens 185 is brought into the illumination beampath 3 by means of drive 190.

All the drives 50, 60, 101, 115, 125, 142, 182, 190 and the detector 200are connected to a data processing system 210 comprising an input/outputunit 215. Control of the mask inspection microscope is effected throughthis data processing system 210. The particular image is stored byreading out of the detector 200; the image data are processed further.

In another exemplary embodiment not shown in the drawings, the maskinspection microscope operates in reflection. Here, the mask 145 isilluminated from the side of the structure 150. The mask thus liesprecisely with its opposite side on the mask holder 140. The radiationreflected from the structure 150 is uncoupled from the illumination beampath 3 in a known manner by means of a beam splitter and continues on,as illustrated in FIG. 2, until it is imaged onto a detector 200.

In a further exemplary embodiment, the mask inspection microscope isoperated with illumination radiation in the EUV range having awavelength of 13.5 nm. The layout is depicted in FIG. 3. The radiationfrom an EUV radiation source 221 is gathered by a collector 222 andreflected via mirrors 224 and 226 to an EUV mask 228 that is to beexamined. A field stop 234 serves to determine the size of theillumination field on the EUV mask 228. Stop 234 is imaged onto themask. A drive 235 serves for focus or defocus the image of the stop 234.Stop 232, is disposed on a stop plate 233, is used to obtain the desiredangle of illumination (analogously to, for example, the stops on thestop plate 45). An image of the EUV mask is projected via the imagingsystem 230 onto a detector 237, which, like drive 235, is connected to anot illustrated data processing system, which reads out the detector andprocesses the image data further. To image the pupil plane onto thedetector 237, an additional mirror (not shown) is brought into the beampath.

Every location on the pupil plane corresponds to radiation of one anglefrom the object plane or the image field plane. Angles, such asillumination angles or diffraction angles, for example, are henceforthstated as corresponding locations on the pupil plane. The locations aregiven in polar coordinates, the pole being the center of the pupil. Theradial coordinates are given in numerical aperture units. The polar axisis on an x-axis, the positive direction in the direction of illuminationbeing defined as to the right. The x-axis and the orthogonal y-axisintersect with the center of the pupil plane. The azimuth angle is theangle between the radius and the x-axis.

The capture of a diffraction pattern is carried out as follows. The mask145 comprising the structure 150 to be examined is placed in the maskinspection microscope on the mask holder 140. By means of drive 142, thedesired slice of the structure 150 is brought into the beam path of themask inspection microscope under control exerted by means of the dataprocessing system 210. The Bertrand lens 185 is disposed in the beampath. By means of drive 50 of the stop plate, the desired illuminationangle or angles are set by selecting a stop 51 on the stop plate 45.

The illumination field on the mask is adjusted via stop 100. In a firstexemplary embodiment, stop 100 is focused on the plane of the mask 145via drive 101. The size of the stop 100 corresponds to the image fieldprojected onto the detector 200 by the objective 160. In anotherexemplary embodiment, the intensity distribution of the illuminationfield is changed by defocusing the image of the stop 100 on the mask145. In another exemplary embodiment, a stop 100 that is larger than theimage field is used. In another exemplary embodiment, the stop 100 isremoved from the beam path.

The diffraction pattern of the structure 150 is projected onto thedetector 200. This image is focused by moving the mask holder 140 bymeans of drive 142 toward the optical axis, i.e., in the z-direction. Itis sufficient if the precision of focus is approximately within a rangeof Δz=3 μm to about 8 μm. The detector 200 configured as a CCD (chargecoupled device) chip is read out by means of the data processing system210 and a digital grayscale image is stored. The performance of themethod with a mask inspection microscope employing illuminatingradiation in the EUV range with a wavelength of 13.5 nm takes placeanalogously.

In an exemplary embodiment of the method during the capture of thediffraction pattern a continuous relative movement between mask anddetector is carried out. This will be accomplished by the movement ofthe mask holder 140 by means of drive 142. The mask 145 is moved in adirection perpendicular to the optical axis 1.

The movement of the mask is performed in such a way, that the wholeregion of the mask to be characterized is scanned. By means of thedetector 200 a single diffraction pattern averaged over the whole regionto be characterized is scanned.

The speed of movements and the exposure time of the detector 200 has tobe specified in a way that each portion of the whole region to becharacterized is moved through the beam path during the exposure time,i.e. the region is scanned.

The movement of the mask is for example accomplished line by line. Thecharacterization is started at a corner of the region, the movement iscarried out parallel to one first outer edge of the region to becharacterized. As soon as the end of the first outer edge will bereached, i.e. as soon as the first line is scanned, the distance of thefirst outer edge will be increased and the movement will be continued inthe opposite direction, that the next line will be scanned.

The region to be characterized may be the whole structured region of themask or a large connected part to be defined freely.

The control of the scanning movement and the analysis of the diffractionpattern is performed by the data processing system 210.

The angle or angles of illumination and the degree of coherence of theilluminating radiation are adjusted by means of the stops 51, 232 in thepupil plane of the mask inspection microscope and by means of the zoomlens 55.

The angle of the illuminating radiation to optical axis I, the z-axis,is adjusted to the grating period of the to-be-examined structure insuch a way that the zeroth and the first (or negative first) diffractionorders are fully contained and resolved in the captured diffractionpattern. The angle of illumination can be adjusted so that higheradjacent diffraction orders, for example the first and seconddiffraction orders, are imaged in the pupil.

The maximum illumination angle in the z-direction for capturing thezeroth and first diffraction orders is limited by the mask-sidenumerical aperture NA_(mask) of the objective. The angle (distance inthe pupil plane) between the maxima of the zeroth and first diffractionorders is found from the wavelength of the illuminating radiation andthe grating period p, in the form λ/p. Examination is possible only ifλ/p<2NA_(mask). In order for the first (or negative first) diffractionorder to fall within the pupil, the value of the illumination angle inthe z-direction must be less than [λ/p−NA_(mask)]. To obtain asymmetrical arrangement of the maxima of the zeroth and firstdiffraction orders relative to the center of the pupil plane, the valueof the illumination angle in the z-direction must be λ/(2p).

In an exemplary embodiment, annular, i.e., ring-shaped, distributions ofillumination angles are employed. In this case, the illumination angleto the z-axis is specified, as noted earlier, while the azimuth angle isleft undefined. Comparable structures on the mask are thus detectedirrespective of their orientation. Examples of stops for practicalimplementations are given in FIG. 4. A first annular stop 61 has a widering, and a second annular stop 62 a narrower ring.

Additional optimized distributions of illumination angles are providedfor different structures. Various structures are usually present on amask. A particularly suitable approach for quality control is to measurethe line widths of structures denoted as x- and y-structures, asmentioned above.

For the examination of x-structures, an illumination angle distributionwith a pole on the x-axis is provided. The azimuth angle of the incidentlight is then 0° or 180°. Corresponding stops (x-monopoles) areillustrated in FIG. 4 on stop plate 45 as reference numerals 52 and 53.The degree of coherence of the first x-monopole 52 is greater than thedegree of coherence of the second x-monopole 53.

For the examination of y-structures, an illumination angle distributionwith a pole on the y-axis is provided. The azimuth angle of the incidentlight is then 90° or 270°. Corresponding stops (y-monopoles) areillustrated in FIG. 4 on stop plate 45 as reference numerals 56 and 57.The degree of coherence of the first y-monopole 56 is greater than thedegree of coherence of the second y-monopole 57.

For the simultaneous examination of x-structures and y-structures, anillumination angle distribution with two poles is provided, designatedhere as an asymmetric xy-dipole. One pole lies on the x-axis and onepole on the y-axis. Such angles of illumination are obtained, forexample, by means of stops where one pole is on the y-axis and one onthe x-axis. Examples are illustrated in FIG. 4 as reference numerals 58and 59.

In a further exemplary embodiment, an illumination angle distribution isused in which a pole lies at the center of the pupil, i.e., theillumination takes place along the optical axis 1. This is advantageouswhen the zeroth, first and negative first diffraction orders of theilluminating radiation diffracted at the structure are contained in thecaptured diffraction pattern. For more precise analysis of thediffraction pattern, the intensities of the first and negative firstdiffraction orders are compared with each other or averaged.

As noted above, captured diffraction patterns are stored as grayscaleimages in the memory of the data processing system 210. These images area matrix of 1000×10000 pixels with intensity values ranging from 0 to255. To determine the intensities of the diffraction maxima, the firststep is to identify their respective positions in the diffractionpattern.

If the nominal value of the grating period is known, this and thespecified illumination angles can be used to calculate the nominalpositions of the to-be-analyzed diffraction maxima of the zeroth andfirst (or negative first) diffraction orders of the particularstructures in the captured diffraction pattern. These calculations andthe further analyses are performed in the data processing system 210.

These nominal positions are used to determine the intensities of thediffraction maxima. This is done by adding up all the intensity valuesof the pixels of the diffraction pattern in a region surrounding thenominal position of a diffraction maximum.

In one exemplary embodiment, the extent of the region of a diffractionmaximum is detected by applying a limit value for the intensity values.For instance, only intensity values that are at least 10% of the maximumintensity within the diffraction maximum are taken into account.Alternatively, a fixed limit is set for the intensity.

In a further exemplary embodiment, the position and the region arefixedly specified for each diffraction maximum.

From the intensities of the first and zeroth diffraction maxima,respectively denoted I₁ and I₀, the quotient I₁/I₀ is calculated; thisis designated the intensity quotient. In a variant of the method, thereciprocal can also be calculated.

In a further exemplary embodiment, the to-be-measured intensities of thediffraction orders are normalized (clear normalization). A region of themask that has no structure of any kind is brought into the imaging beampath of the mask inspection microscope. The diffraction pattern capturedin this way consists solely of an image of the pole or poles of thestops used. The intensities of these poles, denoted as clear intensitiesI_(clear), are determined as described above. Intensities measured fromdiffraction patterns are divided by the clear intensities in order toperform the normalization. The clear intensities for the zeroth andfirst diffraction orders are: I_(0clear)=I₀/I_(clear);I_(clear)=I₁/I_(clear). The clear-normalized intensity values of thediffraction maxima are calculated, for example, in order to compare theintensity values of the diffraction maxima of different diffractionpatterns with one other.

In one exemplary embodiment, in order to calibrate for one or morestructures of a mask whose grating periods and line widths are known, asuitable stop is selected and the intensity quotient is determined.

If the grating period is constant, the intensity quotient is, in goodapproximation, dependent only on line width. Calibration makes itpossible to determine the absolute values of the line widths frommeasurements of the intensity quotients.

The calibration can be performed against absolute measurements performedon the structure on-mask or on-wafer. A scanning electron microscope isused to measure the absolute dimensions of the line width both on themask and on the wafer.

Since only a slight deviation of line width (i.e., of CD, criticaldimension) is to be expected with the masks to be inspected, a linearrelationship between the intensity quotients and the line width can beassumed in good approximation.

A nominal value for the critical dimension is usually known for a maskthat is to be inspected. For purposes of analyzing the intensityquotients, it can be assumed that this critical dimension approximatelycorresponds to the mean of the intensity quotients of comparablestructures. The relative deviation of the intensity quotients from themean then corresponds to the relative deviation from the specifiedcritical dimension.

To characterize a mask, a stop is selected that corresponds to thedesired distributions of illumination angles—x-monopole, y-monopole,asymmetric xy-dipole or annular—and is adjusted to the grating period ofthe structure that is to be examined.

To the extent that the positions on a mask of regions containingcomparable structures are known, the diffraction patterns are capturedspecifically at those positions. The respective intensity quotients arecalculated directly for all the images. The positions of comparablestructures can be determined, for example, from the mask design, i.e.,the data relating to the structure represented on the mask.

If the mask design, i.e., the structure represented on the mask, isknown, the diffraction patterns of the measured positions can also besimulated. To perform the analysis, the intensity quotients of themeasured intensities are compared to those of the simulated intensities.The percentage deviations are represented graphically, as stated above.

In a further measure, for example in cases where the mask design is notspecified, positions distributed evenly over the mask are specified formeasurement.

All the captured diffraction patterns are searched for diffractionpatterns of comparable structures. One criterion is the position of thefirst diffraction maximum, i.e., its distance from the zerothdiffraction maximum and its azimuth angle. Diffraction maxima ofx-structures lie, for example, on the x-axis. A further criterion is theclear-normalized intensities of the diffraction maxima. A furthercriterion is the spread of the diffraction maxima. A further criterionis the difference from the diffraction patterns simulated on the basisof the mask design. These criteria can be applied individually or incombination.

First, taking all the diffraction patterns into consideration, theclear-normalized intensities of the zeroth and all other diffractionmaxima of the diffraction patterns are determined, together with theirdistances from the zeroth diffraction order. Diffraction patterns whosediffraction maxima are at comparable distances are combined into sets. Atolerance range is defined for the distances within a set. As a furthercriterion, it is determined whether the clear-normalized intensityvalues of the respective diffraction orders of different diffractionpatterns lie within a defined tolerance range. Diffraction patterns withintensity values outside the tolerance range are not included in thecharacterization of the mask. An analogous procedure is used for thespatial spread of the diffraction maxima. Diffraction patterns withdiffraction maxima whose spread does not lie within a tolerance rangeare not included in the evaluation.

The intensity quotients are then calculated for each set of diffractionpatterns, as described above.

For the analysis, the mean of all the intensity values is calculated,together with the percentage deviation of all the values from this mean.Different percentage deviations are assigned different colors or colorshades. The measurement values are then represented by the respectivecolor in a two-dimensional diagram of the mask. To the extent that theintensity quotients have been calibrated against known CD values asnoted above, the absolute CD values can be given in the two-dimensionaldiagram.

A further exemplary embodiment is used for masks with numerous regionscontaining identical structures, so-called dies. To increase theprecision, the mean of the intensity quotients of identical positions onall the dies is measured. For the analysis, the percentage deviation ofthe individual intensity quotients from the mean across all the dies isthen given for each position.

An overview of the method according to the invention is provided in FIG.6. Referring to FIG. 6, the method includes selecting the illuminationsetting as a function of structure: x-monopole, y-monopole, asymmetricxy-dipole or annulus, each adjusted to the grating period of thestructure, and specify the intensity distribution of the illuminationfield. The method includes specifying the positions to be measured onthe mask, in which the positions can be distributed in an even grid overthe mask or positions of comparable structures can be selected from themask design. If the positions are distributed in an even grid over themask, the following are performed: for each position, capturing adiffraction pattern; determining the intensities of the zeroth and firstdiffraction orders of the diffraction patterns; and identifyingcomparable structures by comparing the positions and intensities of thediffraction orders. If positions of comparable structures are selectedfrom the mask design, the following are performed: for each position,capturing a diffraction pattern; and determining the intensities of thezeroth and first diffraction orders of the diffraction patterns. Themethod further includes calculating the intensity quotients.

The invention claimed is:
 1. A method for characterizing a mask having astructure, comprising: illuminating said mask under at least oneillumination angle with monochromatic illuminating radiation, so as toproduce a diffraction pattern of said structure that includes at leasttwo maxima of adjacent diffraction orders, capturing said diffractionpattern, determining the intensities of the maxima of the adjacentdiffraction orders, and determining an intensity quotient of theintensities; wherein said diffraction pattern is projected entirely ontoa single detector in order to be captured, said diffraction pattern isbeing captured during a continuous relative movement between said maskand said detector, and the mask is moved relative to an optical axisduring capture of the diffraction pattern.
 2. The method as in claim 1,comprising: determining a line width of said structure from at least onecorrelation between at least one determined intensity quotient of astructure and a known line width of said structure.
 3. The method as inclaim 1, wherein said illuminating radiation generates on said mask anillumination field having a main region and an edge region, said edgeregion surrounding said main region, wherein the intensity of saidilluminating radiation is constant in said main region but decreasescontinuously in said edge region.
 4. The method as in claim 3, whereinthe decrease in intensity in said edge region corresponds to a Gaussianfunction.
 5. The method as in claim 1, wherein said illumination fieldon said mask is defined by means of a field stop and the intensitydistribution of said illumination field is adjusted by defocusing saidfield stop.
 6. The method as in claim 1, wherein said at least oneillumination angle is adjusted to a grating period of said structure. 7.The method as in claim 1, wherein said at least one illumination angleis adjusted to an orientation of the grating period of said structure onsaid mask.
 8. The method as in claim 1, wherein said structure on saidmask is illuminated simultaneously under at least two illuminationangles, each of said illumination angles being adjusted to theorientation of the grating periods of different structures on said mask.9. The method as in claim 1, wherein said at least one illuminationangle is predefined by arranging a stop in a pupil plane of anillumination beam path of a mask inspection microscope.
 10. The methodas in claim 9, wherein said stop is configured as an annulus that isadjusted to the grating periods of different structures on said mask.11. The method as in claim 9, wherein said stop is configured as amonopole, the position of the pole being adapted to the orientation ofthe grating period of a structure on said mask.
 12. The method as inclaim 9, wherein said stop is configured as an asymmetric dipole, thepositions of the two poles being adapted to the orientations of thegrating periods of different structures on said mask.
 13. The method asin claim 1, wherein the intensities of the adjacent diffraction ordersare normalized to a reference value, particularly to the intensity,measured during the characterization of a mask with no structure. 14.The method as in claim 1, wherein a further characterization of thestructure is performed by applying at least one of the criteria:position of the diffraction order maxima, spacing between thediffraction order maxima, intensities of the diffraction order maxima,or spread of the diffraction order maxima.
 15. The method as in claim 1,wherein a further characterization of the structure is performed bycomparing the captured diffraction pattern to a simulated diffractionpattern.
 16. The method as in claim 1, comprising specifying pluralpositions on the mask at which structures will be characterized.
 17. Themethod as in claim 16, wherein positions that are evenly distributedover the surface of the mask are specified.
 18. The method as in claim16, wherein comparable structures are identified in the diffractionpatterns by applying at least one of the following criteria: positionsof the diffraction order maxima, spacing of the diffraction ordermaxima, intensities of the diffraction order maxima, or spread of thediffraction order maxima.
 19. The method as in claim 15, whereinpositions at which comparable structures are formed on the mask arespecified.
 20. The method as in claim 19, wherein the mean of theintensity quotients of all comparable structures and the percentagedeviation of the individual intensity quotients from the mean arecalculated.
 21. A mask inspection microscope comprising a dataprocessing system, in which the mask inspection microscope performs thesteps according to the method as set forth in claim
 1. 22. The method ofclaim 1 in which the mask is moved in a direction perpendicular to theoptical axis during capture of the diffraction pattern.
 23. The methodof claim 1 in which only a portion of the mask is illuminated at anygiven time during capture of the diffraction pattern.